NdFeB system sintered magnets were discovered by Sagawa (one of the present inventors) and other researchers in 1982. NdFeB system sintered magnets exhibit characteristics far better than those of conventional permanent magnets, and can be advantageously manufactured from raw materials such as Nd (a kind of rare-earth element), iron, and boron, which are relatively abundant and inexpensive. Hence, NdFeB system sintered magnets are used in a variety of products, such as driving motors for hybrid or electric cars, battery-assisted bicycle motors, industrial motors, voice coil motors used in hard disks and other apparatuses, high-grade speakers, headphones, and permanent magnetic resonance imaging systems. NdFeB system sintered magnets used for those purposes must have a high coercive force HcJ, a high maximum energy product (BH)max, and a high squareness ratio SQ. The squareness ratio SQ is defined as Hk/HcJ, where Hk is the absolute value of the magnetic field when the magnetization value corresponding to a zero magnetic field is decreased by 10% on the magnetization curve extending across the boundary of the first and second quadrants of a graph with the horizontal axis indicating the magnetic field and the vertical axis indicating the magnetization.
One method for enhancing the coercive force of a NdFeB system sintered magnet is a “single alloy method”, in which Dy and/or Tb (the “Dy and/or Tb” is hereinafter represented by “RH”) is added to a starting alloy when preparing the alloy. Another method is a “binary alloy blending technique”, in which a main phase alloy which does not contain RH and a grain boundary phase alloy to which RH is added are prepared as two kinds of starting alloy powder, which are subsequently mixed together and sintered. Still another method is a “grain boundary diffusion method”, which includes the steps of creating a NdFeB system sintered magnet as a base material, attaching RH to the surface of the base material by an appropriate process, (such as application or vapor deposition), and heating the magnet to diffuse RH from the surface of the base material into the inner region through the boundaries inside the base material (Patent Document 1).
The coercive force of a NdFeB sintered magnet can be enhanced by any of the aforementioned methods. However, it is known that the maximum energy product decreases if RH is present in the main-phase grains inside the sintered magnet. In the case of the single alloy method, since RH is mixed in the main-phase grains at the stage of the starting alloy powder, a sintered magnet created from that powder inevitably contains RH in its main-phase grains Therefore, the sintered magnet created by the single alloy method has a relatively low maximum energy product while it has a high coercive force.
In the case of the binary alloy blending technique, the largest portion of RH will be held in the boundaries of the main-phase grains. Therefore, as compared to the single alloy method, the technique can suppress the decrease in the maximum energy product. Another advantage over the single alloy method is that the amount of use of the rare metal, i.e. RH, is reduced.
In the grain boundary diffusion method, RH attached to the surface of the base material is diffused into the inner region through the boundaries liquefied by heat in the base material. Therefore, the diffusion rate of RH in the boundaries is much higher than the rate at which RH is diffused from the boundaries into the main-phase grains, so that RH is promptly supplied into deeper regions of the base material. By contrast, the diffusion rate from the boundaries into the main-phase grains is low, since the main-phase grains remain in the solid state. This difference in the diffusion rate can be used to regulate the temperature and time of the heating process so as to realize an ideal state in which the RH content is high only in the vicinity of the surface of the main-phase grains (grain boundaries) in the base material while the content of the same is low inside the main-phase grains. Thus, it is possible to further minimize the decrease in the maximum energy product (BH)max than in the case of the binary alloy blending technique while enhancing the coercive force. Another advantage over the binary alloy blending technique is that the amount of the rare metal, i.e. RH, used is reduced.
There are two kinds of methods for producing NdFeB system sintered magnets: a “press-applied magnet-production method” and a “press-less magnet-production method.” In the press-applied magnet-production method, fine powder of a starting alloy (which is hereinafter called the “alloy powder”) is put in a mold, and a magnetic field is applied to the alloy powder while pressure is applied to the alloy powder with a pressing machine, whereby the creation of a compression-molded body and the orientation of the same body are simultaneously performed. Then, the compression-molded body is removed from the mold and sintered by heating. In the press-less magnet-production method, alloy powder which has been put in a predetermined filling container is oriented, and sintered as it is held in the filling container, without undergoing the compression molding.
The press-applied magnet-production method requires a large-size pressing machine to create a compression-molded body. Therefore, it is difficult to perform the process in a closed space. By contrast, in the press-less magnet-production process, which does not use a pressing machine, the processes from the filling through the sintering can be performed in a closed space.